Coherent optical communication systems are known in which an optical signal is transmitted on an optical fiber from a transmitter to a receiver. In the receiver, the optical signal or a portion thereof is mixed with a local oscillator optical signal and converted to an analog electrical signal by photodetector circuitry. The analog signal may then be amplified or otherwise processed and then sampled by analog-to-digital conversion (ADC) circuitry to supply corresponding digital samples. The digital samples may then be supplied to a digital signal processor (DSP), including serializer-deserializer (SERDES) circuitry that may provide a serial output data stream corresponding to data carried by the optical signal.
Typically, the optical signal carries data as a series of bits of information, and these bits are grouped into symbols, such that a series of such symbols are received by the receiver. Each symbol is transmitted over a given time frame referred to as a symbol period (Ts), and the rate at which the symbols are transmitted is 1/Ts and may also be referred to as the symbol frequency or baud rate (fbaud). Often, the timing of the ADC sampling (or the sampling frequency or sampling rate) is such that multiple samples, such as two, are taken during the symbol period in order to adequately detect or recover each symbol, for example, in accordance with the so-called Nyquist Theorem. Accordingly, the ADC sampling is preferably adjusted in accordance with a clock signal, which is timed so that the two samples are taken during each symbol period, instead of, for example, the samples being taken from different symbol periods. The clock signal may also be used to time the input of the digital samples to the SERDES circuitry, so that the samples may be processed in a synchronized manner.
As generally understood, the optical signal may be subject to various impairments during transmission, such as chromatic dispersion (CD), in which different frequency components of the optical signal may propagate at different speeds along the optical fiber. As a result, a portion of the optical signal associated with a preceding symbol may be received at the receiver at the same time as another portion of the optical signal associated with a succeeding symbol, thereby resulting in errors in the detected data. Accordingly, known techniques may be implemented in the DSP to correct or compensate for CD. In one such technique, a known Fast Fourier transform (FFT) circuit is provided to convert the digital samples into frequency domain data including frequency components, which may be appropriately filtered with a known finite-impulse-response (FIR) filter to reduce or eliminate those frequency components associated with CD. The frequency domain data may then be converted back to time domain data with a known inverse FFT (IFFT) to supply time domain, chromatic dispersion compensated, data to the SERDES. Processing of frequency domain data, as noted above, is known to have certain advantages.
Phase detector circuits that process time domain data to determine a phase between the clock signal and the sampling frequency are known. For example, such phase detector circuits may implement a so-called Gardner algorithm. Since FFT circuits may be readily implemented, it would be beneficial to realize a computationally efficient phase detector circuit that operates on frequency domain data supplied by such FFT circuits.